1. Field of the Invention
The present invention relates to a scanning beam control system in which a photobeam is modulated in accordance with an image signal while being deflected in order to scan a photosensitive material for each scanning line. In particular, the present system is suitable for an image recorder having a rotary optical deflector or a vibrating optical deflector such as a polygon mirror, a galvano mirror, a hologram disc or the like in order to conduct photolithography, photocopying, a process of producing printed circuit boards or the like.
In the present invention, attention is mainly directed to an improvement for automatically correcting, or compensating for deviation and nonlinearity of scanning lines in a subscanning direction that is orthogonal to a main scanning direction in which the photobeam is deflected by the optical deflector.
The present invention is also directed to an optical scale structure useful to the photobeam controlling system.
2. Description of Background Art
In scanning beam control systems having rotary or vibrating optical deflectors, a photobeam is deflected in a main scanning direction while being relatively progressed in a subscanning direction in order to two-dimensionally scan an imaging plane such as the surface of a photosensitive material. Accordingly, the quality of an image reproduced on the imaging plane depends on the linearity in the traces of the photobeam on the imaging plane, and it is required to maintain the linearity for obtaining a reproduced image at high quality.
However, since optical deflectors currently available have errors in deflecting a photobeam, the traces of the photobeam on the imaging plane deviate from designated scanning lines in the subscanning direction and desired traces of the photobeam on the imaging plane cannot be obtained without correcting or compensating the errors in the optical deflectors. Therefore, correction is required of the errors, such as inclination correction of mirror surfaces for polygon mirrors, wobbling correction of a mirror surface for galvano mirrors and correction of scan bow for hologram discs, for example.
U.S. Pat. No. 4,661,699 discloses a scanning beam control system employing a polygon mirror as an optical deflector in which the correction of these errors is attained. As shown in FIG. 14, the conventional system employs a ladder pattern or an optical grating structure in which plural pairs of a transparent area T.sub.p and an opaque area S.sub.d are linearly arrayed in the main scanning direction X such that respective tapered portions of the areas T.sub.p and S.sub.d are coupled to each other. The time span during which the photobeam passes through the transparent area T.sub.p in the main scanning direction X is detected and the shift of the photobeam path from a designated path in the subscanning direction Y is found according to the following principle:
Since the boundary between the transparent area T.sub.p and the photo-blocking area S.sub.d is inclined at the neighborhood of a reference line Y=0 defining a scanning line or a designated path of the photobeam, the time span for the photobeam to pass through the transparent area T.sub.p depends on the Y-coordinate of the photobeam path on the optical grating structure. For example, the time span required for the photobeam to pass thruogh the transparent area T.sub.p along a line Y=d as shown in FIG. 14 with a series of beam spots BS.sub.d1, BSd.sub.2 and BS.sub.d3 is shorter than that for the reference line Y=0 as shown with another series of bean spots BS.sub.01, BS.sub.02 and BS.sub.03. Therefore, when the quantitative change of the photobeam penetrating through the transparent area T.sub.p is detected and converted into an electric pulse, the shift d can be estimated by comparing the width of the electric pulse with that for the case where the photobeam scans along the reference line Y=0.
However, the conventional system is employable only in image recorders in which a main scanning speed on a scanned plane is constant, such as an image recorder whose imaging optical system has a polygon mirror and an f.multidot..theta. lens. The reason is as follows:
In an image recorder having galvano mirror in place of a polygon mirror, for example, the width of the electric pulse obtained through the transparent area T.sub.p varies even if the photobeam path is not deviated from the reference line, since the main scanning speed on the scanned plane is not constant in one swing of the galvanometer mirror. In other words, the shift in the pulse width reflects not only the deviation of the photobeam path from the reference line but also the temporary change of the scanning speed. Consequently, if the conventional system is employed in such an image reorder, the scanning lines are undesirably bent on the scanned plane.
The foregoing are problems caused in general in image recorders in which the deflection speed of the photobeam is not constant. Other examples having this character are image recorders having hologram discs as photobeam deflectors, and those having f.multidot.sin .theta. lenses or f.multidot.tan .theta. lenses in place of the f.multidot..theta. lenses. In the latter examples, the main scanning speed varies depending on the projection angle of the photobeam. In particular, when the galvano mirror is employed as a deflector, the curvature of the scanning lines varies for each scanning line since the scanning speed varies depending on subscanning coordinates as well as the main scanning coordinates, so that the scanning control often brings additional disturbance to the parallel array of scanning lines instead of correcting the shift of the scanning lines or the photobeam path.
Furthermore, even if a polygon mirror is employed, a precise scanning control cannot be attained in the case where the rotational speed of the polygon mirror is not uniform and has jitters. Similarly, if the scanned plane is considerably wide, the conventional control system as applied to an image recorder having an f.multidot..theta. lens does not give straight scanning lines, since the scanning speed is not uniform over the scanned plane.
Therefore, the conventional system is quite restricted in its application.
Furthermore, since the change of the pulse width due to the shift of the photobeam path in the subscanning direction is very small, the sensitivity of the shift in the conventional system is low even if the system is employed in an image recorder whose scanning speed is constant.
Additionaly, the pulse width becomes narrow as the imaging speed is increased in image recorders such as a laser plotter, and therefore, the detection of the pulse width is often influenced by electoric noises generated in electronic circuits such as a comparator.